1. Field of the Invention
The present invention relates to a method of and an apparatus for inspecting a honeycomb-shaped object having a plurality of small through holes defined therein, and more particularly to a method of and an apparatus for inspecting a honeycomb-shaped object made of ceramics to determine whether there are interstices developed therein or not.
2. Description of the Prior Art
Honeycomb-shaped objects made of ceramics having a plurality of small through holes defined therein tend to have interstices such as internal cracks or voids developed in thin partitions defining the through holes when the honeycomb-shaped objects are manufactured. The interstices develop due to a difference of contraction between the peripheral portion and the inner portion of the honeycomb-shaped object, as the object starts contracting from the periphery thereof when the object is subjected to a drying process or a sintering process after the object has been formed. If an inspected honeycomb-shaped object is found to have a number of interstices in excess of a predetermined limit, it is discarded as being susceptible to functional or durability problems.
However, since the diameter of the through holes is very small and the partitions that define the through holes are thin, it is highly difficult to confirm the presence of any interstices in inner partitions through a visual inspection process, though it is relatively easy to visually confirm defective interstices in the vicinity of both ends of the honeycomb-shaped object.
According to a Japanese laid-open patent publication No. 58-155343 and a U.S. Pat. No. 4,319,840, there has been proposed a process of inspecting a honeycomb-shaped object for any interstices therein by applying a parallel light beam to the honeycomb-shaped object, inclining the axis of the through holes in the honey-comb-shaped object to the optical axis of the parallel light beam through a certain angle to introduce the parallel light beam into the through holes, and detecting any light beam that has passed out of the through holes to determine whether there are interstices in the honeycomb-shaped object. More specifically, when the honeycomb-shaped object is tilted through the angle, the light beam passing straight through the through holes is interrupted, and any light beam passing through interstices is projected onto a screen. Therefore, it can be determined whether there are interstices in the honeycomb-shaped object by confirming any light beam projected onto the screen.
However, a certain pattern is formed on the screen by dispersed light that is reflected by inner wall surfaces of the through holes, other than the light beam passing straight through the through holes. Such a pattern cannot easily be distinguished from the pattern which is formed on the screen by any light beam that has passed through interstices in the honeycomb-shaped object. Furthermore, where the through holes are slightly curved, the light beam passing straight through the through holes may be applied to the screen depending on the angle between the axis of the through holes and the optical axis of the light beam, and may not be distinguished from any light beam that has passed through interstices in the honeycomb-shaped object. As a result, it may not be possible to determine whether honeycomb-shaped object contains any undesirable interstices.